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I saw a pair of 10" rally wheels at Carlisle a few weeks back. It was pretty obvious because they were right next to a set of 8" rally's. It was also obvious that the wheels had been cut and a 2" ring inserted on the outside. They were done very well and would no doubt work fine, but the additional space had been added to the outside.
I would guess that any competent machine shop could do this for you with good results. You just need to decide where you want the extra width added, inside, outside or some on both sides. If you're wanting to use 15" rally's, adding anything close to an inch to the inside is probably going to require off-set trailing arms.
You can't change the rear offset on vette rear wheels without going to offset trailing arms. The stock offset is 3 3/4" for vettes, it's 4" on other GM wheels. Anymore rear offset causes clearance problems with the e-brake cable bracket on the trailing arm.